Slabstock polyurethane foams having relatively low gross densities are foamed by carbon dioxide produced in situ during the isocyanate/water reaction or by a combination of carbon dioxide formed in situ and organic blowing agents, such as halogenated hydrocarbons, added to the reaction mixture. Foams having gross densities below about 27 kg/m.sup.3 can be produced by the process of the present invention, whereas foams having gross densities below about 21 kg/m.sup.3 are generally produced by addition of auxiliary blowing agents. The different procedures are used because the quantity of water otherwise necessary for the lower density foams can often lead to serious discoloration or, in extreme cases, even to spontaneous ignition of the foam slabs caused by the highly exothermic nature of the isocyanate-water reaction. In addition, with certain polyols the use of large quantities of water (for example, 4.5 parts) results in relatively hard and brittle foams instead of the soft and hypersoft materials often required.
All auxiliary blowing agents hitherto used, such as halogenated hydrocarbons, carbon monoxide from the AB process, low-boiling hydrocarbons, and the like, produce serious pollution of, and thus endanger, the workplace and the environment. Despite many attempts, no environmentally safe blowing agent has hitherto been produced.
It has now surprisingly been found, however, that under certain conditions water can act both as a chemical blowing agent (that is, by producing carbon dioxide from the isocyanate-water reaction) and as a physical blowing agent. Consequently, water alone may be used as the blowing agent for producing foams having gross densities below 21 kg/m.sup.3. In particular, the process of this invention uses an atypically large quantity of water relative to the "base polyol" (at least 5 parts by weight water per 100 parts polyol) and, at the same time, a hitherto prohibitively low isocyanate index of less than 80.
British Patent No. 892,776 indicates that rigid or semirigid polymer foams can be obtained by reaction of 100 parts by weight of a monomeric organic polyisocyanate with from 1 to 30 parts by weight of water in the presence of a surface-active agent and a catalyst, preferably in the presence of from 5 to 30 parts by weight of a polyfunctional compound containing two or more isocyanate-reactive groups. However, the "catalysts" described in this reference are inorganic materials of the potassium acetate or sodium hydroxide type. Thus, the foaming process follows a course largely determined by trimerization reactions and the principle of maintaining low isocyanate index, as used in the present invention, does not come into effect. In addition, the flexible slabstock polyurethane foams produced in accordance with the present invention differ from the rigid and semirigid foams obtainable according to the British patent, particularly in exhibiting different deformation characteristics. For example, deformations observable during hardness measurements indicate that the flexible slabstock polyurethane foams produced in accordance with the present invention have higher resilience (that is, faster recovery after loading) and a lower hardness level, two advantageous characteristics in view of performance requirements. The value characteristic of the deformation property is relative energy absorption after 70% compression, H 70, as shown in Table 1 below.